Motor vehicle generators may include a diode bridge circuit, by which the alternating voltage generated by the generator is rectified.
Diode rectifiers may be relatively robust and reliable, and are consequently particularly suitable for operation in a motor vehicle. They do, however, have certain disadvantages too. In particular, diodes demonstrate a voltage drop in the forward direction that is not negligible. Zener diodes, particularly ZR diodes, are used for the rectifiers, which normally have a forward voltage of ca. 0.7 V. Along with the intermittently large currents of more than 200 A, this leads to a power loss at the diode of up to 50 W (in the case of three-phase generators). As a result, the diodes have to be designed for this sort of power, and especially also have to be sufficiently cooled.
Diodes having a lower forward voltage are known, such as germanium diodes or Schottky diodes, but these cannot be used in the vehicle field without any problem, since especially their characteristic temperature dependence makes using them impossible in known power supply units for motor vehicles, for the following reasons:
At the start of the vehicle, the field controller or charge controller, fed by the starter battery, supplies current to the exciter winding of the generator using a so-called preexcitation current. As soon as the rotor of the generator is turning, the field regulator is able to detect a voltage signal at one phase (in the following phase signal) and, from its frequency, derive the generator rotational speed. When a specified cut-in speed is reached, the full excitation current is then started.
In the usual power supply units for the vehicle electrical system of a motor vehicle, however, the a.c. component of the phase signal has to have a certain minimum voltage. In addition, the d.c. component of the phase signal may not exceed a certain threshold. The minimum required phase alternating voltage may amount to 3 V, for example, measured from peak to peak, and the maximum admissible d.c. current component 8 V, for example.
During the preexcitation clocking, when the generator is not yet supplying any current into the vehicle electrical system, the d.c. current value and the a.c. voltage value of the phase voltage are influenced by the blocking-state currents of the rectifier diodes. Now, if the abovementioned diodes are used, which demonstrate a characteristic temperature dependence, in the heated state, for example, at a time after shutting down the internal combustion engine, there is a different voltage drop at each diode. Therefore, in the extreme case, the d.c. portion of the phase voltage may assume values outside an admissible range, so that the field regulator, for example, at a starting process in the heated state, does not detect, or detects too late the starting rotational speed and does not switch on, or switches on the generator too late.
It is therefore believed to be desirable to provide a power supply unit for a vehicle electrical system of a motor vehicle, which is suitable for the use of temperature-dependent rectifier elements.